September 2021

Bright Sky Digest 

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Stargazing in Seattle

Bob Mulford

September-October 2021

 
Full moon in September, the Harvest Moon, occurs Monday September 20 at 4:55 AM PDT. The Fall equinox occurs shortly after noon on Wednesday September 22.
 
On September 15, civil twilight begins about 7:50 PM and civil dawn begins about 6:15 AM. At this time of year, daytime is growing noticeably shorter day by day. By the end of September, civil twilight arrives about 7:20 PM and civil dawn is already brightening the sky by 6:00 AM.
 
Venus remains low in the southwest, about eleven degrees elevation at Sunset during September and October. Venus currently shines at a brilliant magnitude of minus-four. With clear sky and an unobstructed view toward the southwest, Venus is bright enough to be easily visible by the beginning of civil twilight, when it stands about seven degrees above the horizon. Look for a thin crescent moon, 2.7 days old, about ten degrees to the right of Venus on the evening of October 8. On the following night, October 9, look for a slightly fatter crescent moon less than four degrees above and to the right of Venus.
 
The constellation Orion is already well above the southeastern horizon by dawn in mid-September. By the beginning of October, the great nebula in Orion (M 42) transits before the beginning of nautical twilight.
 
This year, September and October are prime months for observing Jupiter. The giant planet reached opposition last month, and is now conveniently placed in the early sky. Normally, the best views of a planet occur when it is highest in the sky, that is, when it transits. In mid-September, Jupiter transits about 11 PM. By mid-October, Jupiter will transit around 9 PM. Jupiter shines at magnitude -2.7 and is easily recognized as the brightest object in the southeast sky during twilight (it will be due south later in the evening when it transits). Binoculars provide enough magnification to see Jupiter and its four Galilean Moons. Jupiter’s moons orbit rapidly, and with a telescope one can watch them move behind the giant planet, or transit in front of Jupiter’s disk. It is hard to see a moon while it is transiting the bright disk of Jupiter, but its shadow is easily seen. You can find predictions of shadow transits, eclipses, visibility of the Great Red Spot, and other interesting Jupiter events at the Shallow Sky web site (https://shallowsky.com/galilean/). A good strategy for observing Jupiter is to check this site or a similar tool before you go out to observe, but here are a few of the many interesting events visible from the Pacific Northwest in late September:

September 17 Io shadow transit begins 2:33 AM and ends 4:52 AM
September 18 Io shadow transit begins 9:03 PM and ends 11:21 PM
September 19 Europa shadow transit begins 9:14 PM and ends 00:06 AM
September 20 Ganymede shadow transit begins 03:50 AM and ends after sunrise
September 25 Io disappears (behind Jupiter) at 00:48 AM
September 25 Europa disappears at 03:40 AM
September 25 Io reappears at 03:58 AM
September 25 Io shadow transit begins 10:58 PM and ends 01:16 AM
  
Times and positions have been adjusted for Seattle’s location (47.6N, 122.3W) and are useful throughout the Pacific Northwest. At civil twilight, the sky is dark blue but not quite dark, and only the brightest stars and planets are visible. At nautical twilight, a suburban sky is about as dark as it gets. There is much to see in the sky even if you don’t have a telescope. EarthSky (https://earthsky.org) has an excellent guide to sky events, and In-the-sky.org (https://in-the-sky.org/newscal.php) lists events in a convenient monthly calendar format. Sky and Telescope (https://skyandtelescope.org/) is another good source of information for both novice and experienced sky watchers.

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